This invention relates to the field of antennas for ground-penetrating radar, and to their use. In particular, it relates to a high-frequency antenna system useful for, among other applications, identifying plastic pipes and rebar in concrete, and to a process using said antenna system to identify plastic pipes and rebar in concrete.
Many applications and needs exist today for ground-penetrating radar systems. Among these, in particular, are a need to probe man-made structures such as bridge decks, to assess their condition or to locate internal features. Needs also exist to probe natural structures and natural structures that have been altered by mankind as, for example, by the burying of metal or plastic pipes or other objects.
Ground penetrating radar (GPR) is a technique that may be used to image the inside of a structure by collecting the echoes (or reflections) resulting from electromagnetic signals such as, for example, electromagnetic waves of typically high frequency, radiated into the structure.
One such structure is a bridge deck. A bridge deck is the portion of a bridge upon which vehicles travel. Bridge decks are typically made of reinforced concrete. As referred to herein, concrete is a mixture of fine and coarse aggregates such as, for example, crushed stone or gravel, firmly bound into a monolithic mass by a cementing agent such as, for example, Portland cement. Reinforced concrete as referred to herein is concrete in which metal rods or bars, preferably made of steel, are incorporated into the concrete in such a manner as to reinforce or strengthen the more or less brittle nature of concrete and the resulting structure. Such rods or bars carry the tension to which a concrete structure may be subjected, thus reinforcing the concrete, and are referred to herein as reinforcing bars or rebars. As used herein, a substantially concrete structure is a structure where the primary constituent is concrete. Such a substantially concrete structure may contain reinforcing bars to improve tensile strength, a waterproofing membrane to protect the structure from moisture, an asphalt layer or overlay, other added elements to improve durability or performance, or possibly inadvertently added elements.
The depth of the rebars relative to the concrete surface, commonly called xe2x80x9cconcrete cover,xe2x80x9d is important for at least two reasons. First, the depth of the rebar affects the overall tensile strength of the bridge deck, and second, rebar corrosion potential is related to the depth of the rebar in the concrete. Rebar corrosion may compromise the structural integrity of a reinforced concrete bridge deck, and lead to further deterioration of the concrete that further compromises structural integrity. Additionally, a bridge deck may be subjected to extreme climates such as, for example, snow, ice, and thermal freeze-thaw cycles. Further, such extreme climates, and human intervention to permit the flow of traffic on the bridge amidst these harsh conditions, may result in the ingress of road salt. These factors may lead to the eventual deterioration of portions of the bridge deck, making travel on the bridge unsafe.
Consequently, at least one government, the State of New Hampshire, USA, has implemented a quality control (QC) policy which rewards bridge contractors who place the rebars at the correct depth in new bridge decks, and penalizes contractors negligent in rebar placement. The QC policy specifies the measurement of rebar cover to within xc2x13 millimeters.
The policy also requires the measurement of many rebars per bridge deck, to establish a statistical basis for assessing contractor performance. The large number of rebars that need to be located makes prohibitive the use of invasive techniques, such as, for example, core drilling, to obtain the depths of all of the rebars.
Thus, there is an established need to accurately determine the position and depth of rebars used in the construction of reinforced concrete bridge decks.
A need also exists for locating plastic pipes in a variety of structures, including structures containing rebars. It may be necessary to locate a plastic pipe to effect a repair or to avoid the plastic pipe when effecting a repair or performing other work on other parts of the structure. Such pipes may conduct fluids or they may serve as conduits through which wires or other cables (e.g., optical fiber communication cables) are strung. Damaging a pipe and the wires and cables inside could not only cause service disruptions, but also may prove dangerous to construction workers.
Thus a need exists for locating both rebars and plastic pipes in structures such as bridge decks. The ability to do so with the same apparatus, facilitating the determination of relative positions of those objects, would have particular value.
In general, the need exists for locating and identifying targets such as rebar and plastic pipe at shallow depths in the ground or concrete or like material. Good horizontal resolution is needed in order to distinguish rebars in concrete. Such rebars are generally closely spaced and cannot be distinguished individually by large antennas. That is, target dimensions usually cannot be resolved much smaller than antenna size but a need exists for resolving closely spaced rebars.
Typically, the rebars inside a reinforced concrete structure are strong radar wave reflectors. Locating rebars within a reinforced concrete structure and determining their depths may be accomplished by analyzing the reflections, particularly the amplitudes and arrival times of the reflections, from the rebars in the reinforced concrete structure.
In a first aspect of the invention, a small ( less than 10 cm) ground-penetrating radar (GPR) antenna is provided, which has two element pairs orthogonally oriented. This is a ground-coupled, dual-channel GPR impulse antenna system working at a center frequency of about 1.5 GHz or higher. Each of the element pairs includes a transmit dipole and a receive dipole. The dipoles are disposed in a small metal box which shields them. The dipoles are dimensioned, shaped and arranged (a) to minimize the mutual impedance between the elements, so the transmitted and the received pulses are not affected by the presence of the second (i.e., other) pair of dipoles; and (b) to accommodate a desired rise time of a transmitter pulse (e.g., a few hundred picoseconds) without reflections destructively interfering with the transmitter signal.
Preferably, the dipoles are disposed in the metal box and their placement as well as the box dimensions are such that (a) reflections from a top of the box reinforce the transmitted signal and (b) the impedance at the feedpoints of the antennas do not change much as the antenna system is moved over the terrain or structure being probed.
The antennas must couple well to soil and like media for the xe2x80x9cgroundxe2x80x9d probing uses discussed herein. Preferably, the antennas also may be disposed and arranged and possess the characteristic that direct wave coupling between transmit antennas and receive antennas is kept relatively low.
Electromagnetic waves with electric field components parallel to the long axis of rebar are preferentially scattered relative to electromagnetic waves with a primary electric field polarization perpendicular to the long axis. The opposite is true for the air-filled plastic and polyethylene pipes commonly found in concrete. An antenna (or, more accurately, antenna system) according to the invention, contains two pairs of transmit-receive co-located antennas that are used to collect two channels of data. The radiated electric fields from the two different transmit-receive combinations are orthogonal to each other. Data are collected with the antenna system by moving it over a series of parallel profile lines to detect the linear targets in concrete. The orientation of targets detected over several profile lines is readily obtained and the polarization directions of the two channels of data are known. From this information, the channel of data containing the largest polarization vector component parallel to the target can be determined. The ratio of the target reflection amplitudes from the two channels of data can then be used to obtain the composition of the target.
In related application Ser. No. 09/457,749, filed Dec. 8, 1999 in the name of Roger L. Roberts, now U.S. Pat. No. 6,429,802, there is described a method of determining, for a substantially concrete structure having at least a first side and containing one or more reinforcing bars, a distance of the one or more reinforcing bars from the first side of the substantially concrete structure. One or more computer-readable data signals are received, and each received data signal represents an electromagnetic signal detected from an area of the concrete structure. One or more of the detected electromagnetic signals include electromagnetic energy reflected from the concrete structure as a result of an electromagnetic signal transmitted into the concrete structure. A distance of one or more of the reinforcing bars from the first side of the substantially concrete structure is determined from the one or more computer-readable data signals. In another embodiment, the foregoing application discloses, for a substantially concrete structure having at least a first side and containing one or more reinforcing bars, a system for determining a distance of the one or more reinforcing bars from the first side of the substantially concrete structure. The system includes a computer-readable storage medium to store one or more computer-readable data signals, wherein each data signal represents an electromagnetic signal detected from the concrete structure, and one or more of the detected electromagnetic signals include electromagnetic energy reflected from the concrete structure as a result of an electromagnetic signal transmitted into the concrete structure, and wherein each detected signal corresponds to an area within the concrete structure. The system also includes a data analysis application to receive the computer-readable data signals and determine from the data signals a distance of one or more of the reinforcing bars from the first side of the substantially concrete structure. An antenna system according to the invention is useful in such systems and for practicing such methods.
According to an aspect of the invention, there is provided an antenna system for ground-penetrating radar use, comprising first and second co-located antenna element pairs orthogonally oriented with respect to each other. Each of the element pairs includes a transmit element and a receive element. A metallic enclosure (such as a box) shields the element pairs and is open on one side to face a structure to be probed by signals from the transmit elements. The transmit and receive elements are dimensioned, shaped and arranged (a) to achieve low mutual impedance between the elements, (b) to have high sensitivity to reflected signals received from the structure being probed; and (c) to accommodate a desired rise time of a transmit pulse. The transmit elements and receive elements preferably are electrical dipoles but they may also be magnetic dipoles such as slot antennas. When electrical dipoles are employed, they may be formed of elongated diamond-shaped conductive surfaces or be of other appropriate geometries.
According to another aspect of the invention, the transmit and receive elements are positioned within the box and the box dimensioned such that (a) reflections from a top of the box reinforce transmitted signals and (b) the impedance at the feedpoints of the transmit and receive elements is relatively stable as the antenna system is moved over the structure being probed.
Preferably, such an antenna system also includes material which is lossy at the frequencies at which the antenna system is operated, at least partly filling the space in the enclosure between the transmit and receive elements and an opposite portion of the enclosure.
According to yet another aspect of the invention, there is provided in the enclosure a conductive shield constructed and arranged to shield the transmit elements from the receive elements.
In accordance with a further aspect of the invention, in operation, data are collected with the antenna system over a series of parallel profile lines to detect the linear targets in concrete. The orientation of targets detected over several profile lines is readily obtained and the polarization directions of the two channels of data are known. From this information, the channel of data containing the largest polarization vector component parallel to the target can be determined. The ratio of the target reflection amplitudes from the two channels of data can then be used to obtain the composition of the target. That is, the two amplitudes can be compared to determine which is greater. That information, together with information as to the orientation of the antenna assembly relative to the target, reveals the nature of the material providing the reflection. The resulting information about rebar and plastic pipe location can be displayed on a computer-driven screen.
These and other features and advantages of the invention will be more readily understood and appreciated from the detailed description below, which should be read together with the accompanying drawing figures.